1. Field of the Invention
The present invention relates to a method of and an apparatus for processing an image, wherein errors between the actual image and expressed image produced in a plurality of pixels forming the image are computed and processed.
2. Description of the Related Art
Each of originals is read or scanned by a scanner unit having an image sensor such as a CCD (Charge Coupled Device) or the like. After data read from the original has been subjected to image processing by an image processing unit, the so-processed data can be printed on a recording sheet by a printing unit such as a TPH (Thermal Print Head) or the like.
A system has been generally used wherein upon printing, a thermal head is moved in a main scanning direction and sub-scanning is performed by feeding a recording sheet.
FIG. 18 shows a thermal head 50 attached to a printing unit, for performing thermal printing. Adjacent heating elements 51 are formed so as to be identical in size to each other. Thus, the image processing at the step prior to the printing is performed as the image processing based on pixels identical in size (area) to each other.
Further, the recording of data on the recording sheet is carried out by selectively driving the respective heating elements of the thermal head. Described specifically, upon image processing, the respective heating elements are energized or de-energized under drive control executed in a binary form to thereby form predetermined characters and images.
When an original, a picture or the like has multi levels of gray in density, the multi levels of halftone can be reproduced on a recording sheet at high gradation by using an error diffusion method in the image processing.
The error diffusion method is used for the purpose of determining errors to be propagated to a pixel to be noted from peripheral pixels to obtain the errors to be propagated to the noted pixel and using the errors to make a decision about the degree of halftone of the noted pixel to thereby achieve high tonal characteristics.
When the decision about the tone of the noted pixel is finally made, a middle density or level in multi levels of tone is set as a threshold, for example and the density of the noted pixel, which is added with the errors, is expressed in a binary form depending on whether the density is higher or lower than the threshold.
Since, however, the conventional thermal head 50 comprises the heat elements 51 identical in size to each other, tones representable by the adjacent two heating elements are limited to three levels of tone. Therefore, the conventional thermal head becomes complicated in structure and is increased in scale to show multi levels of halftone.
Thus, a thermal head 60 constructed so that adjacent heating elements are alternately varied in size as shown in FIG. 19 has been known. According to the thermal head 60, the adjacent heating elements 61 and 62 are different in heating value from each other. Therefore, four levels of halftone can be expressed in total by the adjacent two heating elements 61 and 62, so that multi levels of halftone can be easily produced.
However, an image processing method, which is performed using an error diffusion method using the thermal head 60, has not yet been proposed.
Namely, it is necessary to perform image processing for the thermal head 60 as well as to use the aforementioned thermal head 60 in order to exhibit a higher tonal characteristic.
However, there has heretofore been proposed only image processing utilizing the error diffusion method used with the thermal head comprised of the heating elements identical in size to each other. This is because distances between the noted pixel and its peripheral pixels may simply be considered due to the fact that the heating elements are identical in size to each other.
Thus, the thermal head 60 comprised of the noted pixels (heating elements) having the two types of sizes cannot cope with or treat the image processing in case the error diffusion method based on the distance between the respective heating elements 61 and 62 alone is used.